Re: [squid-users] (53) Software caused connection abort

This message seems to indicate you have clients who aborted the
connection before it was even fully established, during the TCP
handshake, and your OS now signals such connection attempts to the
application with an error.

A tcpdump should tell you pretty quickly if this is indeed the case.
Simply run 'tcpdump "your.proxy.ip.address" "proxy_port"' for a short
while until you see one of these messages, and then look for RST
packets in the dump.